Optical-electric printed wiring board, printed circuit board, and method of fabricating optical-electric printed wiring board

ABSTRACT

An optical-electric printed wiring board includes an electric wiring substrate having electric interconnects, and an optical wiring layer stacked on the electric wiring substrate and having a surface on which an optical part is mounted. The optical wiring layer includes a core for propagating light, a clad for sandwiching the core, and a mirror for reflecting light propagating in the core toward an optical part mounted on the optical wiring layer, or reflecting light from an optical part into the core. The electric wiring substrate includes conductive setting portions each of which extends through the optical wiring layer in the direction of stacking and has an end face on which an optical part is set. These conductive setting portions obtain electrical conduction between the optical part and the electric interconnects.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a Continuation Application of PCT Application No.PCT/JP00/03440, filed May 29, 2000, which was not published under PCTArticle 21(2) in English.

[0002] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Applications No. 11-150460, filed May 28,1999; and No. 11-150461, filed May 28, 1999, the entire contents of bothof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to an optical-electric printedwiring board having both optical interconnects and electricinterconnects, a method of fabricating the same, and a printed circuitboard fabricated by mounting an optical part or an electric part on theoptical-electric printed wiring board.

[0005] 2. Description of the Related Art

[0006] Recently, the degree of integration of transistors is increasing,among other electric elements such as semiconductor large-scaleintegrated circuits (LSIs). Some transistors have an operating speed of1 GHz as a clock frequency.

[0007] To mount these highly integrated electric elements on an electricprinted wiring board, packages such as a BGA (Ball Grid Array) and a CSP(Chip Size Package) have been developed and put into practical use.

[0008] Generally, as the internal clock frequency of electric elementsrises, the external inter-element signal speed of these electric elementrises. This high-speed, inter-element signal produces noise such asreflection caused by poor shapes of electric interconnects connectingthe elements, or the influence of crosstalk. In addition, the high-speedinter-element signal generates many electromagnetic waves from theelectric interconnects to adversely affect peripheral circuits.Therefore, present systems are constructed by lowering this signal speedbetween electric elements to the extent at which no such problems arise.In this case, however, the functions of highly integrated electricelements are not fully utilized.

[0009] To solve these problems, copper electric interconnects on aprinted circuit board are partially replaced with optical interconnectssuch as optical fibers or optical waveguides, and optical signals areused in place of electrical signals. This is because optical signals cansuppress the generation of noise and electromagnetic waves.

[0010] From the viewpoint of high-density mounting and miniaturization,it is desirable to fabricate an optical-electric printed wiring board bystacking electric interconnects and optical interconnects on the samesubstrate. However, when an optical part such as a laser light emittingelement or light receiving element is to be mounted on a conventionaloptical-electric printed wiring board, it is difficult to opticallyalign the optical axis of this optical part with that of an opticalinterconnect. Generally, only a skilled operator can align these opticalaxes. Accordingly, compared to electric parts which can be automaticallysoldered by a reflow furnace or the like, mounting optical parts on anoptical-electric printed wiring board is very expensive.

BRIEF SUMMARY OF THE INVENTION

[0011] The present invention has been made in consideration of the abovedrawbacks of the prior art, and has as its object to provide anoptical-electric printed wiring board which can realize high-densitymounting or miniaturization, and which can mount an optical part and anelectric part with high accuracy, to provide a method of fabricating thesame, and to provide a printed circuit board mounting an optical part oran electric part on the optical-electric printed wiring board.

[0012] One aspect of the present invention is an optical-electricprinted wiring board comprising an electric wiring substrate having anelectric interconnect, and an optical wiring layer stacked on theelectric wiring substrate and having a surface on which an optical partis mounted, characterized in that the optical wiring layer comprises acore for propagating light, a clad for sandwiching the core, and amirror for reflecting light propagating in the core toward the opticalpart, or reflecting light from the optical part into the core, and theelectric wiring substrate comprises conductive setting means which is aconductor column extending through the optical wiring layer in thedirection of stacking and having an end face on which the optical partto be mounted is set, the conductive setting means obtaining electricalconduction between the optical part to be mounted and the electricinterconnect.

[0013] Another aspect of the present invention is a printed circuitboard fabricated by mounting an optical part or an electric part on theoptical-electric printed wiring board described above.

[0014] Still another aspect of the present invention is a method offabricating an optical-electric printed wiring board, characterized bycomprising the steps of forming conductive setting means on apredetermined electric interconnect of an electric wiring substrate,coating the electric wiring substrate with a first cladding layer,coating the first cladding layer with a core layer, coating a portion ofthe first cladding layer and the core layer with a second cladding layerto obtain an optical wiring layer, exposing an end face of theconductive setting means from the optical wiring layer, forming anelectric interconnect on the optical wiring layer, and forming a mirrorin a predetermined position of the optical wiring layer by perforation.

[0015] With the arrangements as described above, it is possible toprovide an optical-electric printed wiring board which can realizehigh-density mounting or miniaturization, and which can mount an opticalpart and an electric part with high accuracy, to provide a method offabricating the same, and to provide a printed circuit board fabricatedby mounting an optical part or an electric part on the optical-electricprinted wiring board.

[0016] Note that embodiments according to the present invention includeinventions in various stages, and diverse inventions can be extracted byproper combinations of a plurality of constituent features disclosed.For example, if an invention is extracted by omitting some constituentfeatures from all constituent features described in the embodiments, theextracted invention is carried out by properly compensating for theomitted portions by well-known conventional techniques.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0017]FIG. 1 is a top view showing, from the optical part mounting side,an optical-electric printed wiring board 10 according to the firstembodiment;

[0018]FIG. 2 is a sectional view taken along a C-C direction in FIG. 1;

[0019]FIGS. 3 and 4 show a printed circuit board mounting an opticalpart on the optical-electric printed wiring board 10 according to thefirst embodiment;

[0020]FIGS. 5A to 50 are sectional views showing the individual steps ofa method of fabricating the optical-electric printed wiring board 10;

[0021]FIG. 6 is a plan view of a portion for mounting an optical part ofthe optical-electric printed wiring board obtained by the secondfabrication method;

[0022]FIG. 7A is a sectional view taken along a C-C direction in FIG. 6;

[0023]FIG. 7B shows a printed circuit board mounting an optical part onthe optical-electric printed wiring board 10 according to the firstembodiment;

[0024]FIG. 7C shows another example of the optical-electric printedwiring board 10 according to the first embodiment;

[0025]FIGS. 8A to 8E are sectional views showing the individual steps ofa method of fabricating an optical wiring layer 11;

[0026]FIGS. 9A to 9E are sectional views showing the individual steps ofa method of fabricating an electric wiring substrate 12;

[0027]FIGS. 10A to 10E are sectional views showing the individual stepsof a method of stacking the optical wiring layer 11 on the electricwiring substrate 12;

[0028]FIG. 11 is a top view showing, from the optical part mountingside, an optical-electric printed wiring board 60 according to thesecond embodiment;

[0029]FIG. 12A is a sectional view taken along a direction C-C in FIG.11;

[0030]FIGS. 12B and 12C are enlarged views showing a recess 51 and itsvicinity;

[0031]FIG. 13 is a top view showing, from the optical part mountingside, the optical-electric printed wiring board 60 according to thesecond embodiment;

[0032]FIGS. 14A to 14E are sectional views showing the individual stepsof a method of fabricating an optical wiring layer 11;

[0033]FIGS. 15A to 15E are sectional views showing the individual stepsof a method of fabricating an electric wiring substrate 12;

[0034]FIGS. 16A to 16E are sectional views showing the individual stepsof a method of stacking the optical wiring layer 11 on the electricwiring substrate 12;

[0035]FIG. 17 is a top view showing, from the optical part mountingside, an optical-electric printed wiring board 62 according to the thirdembodiment;

[0036]FIG. 18 is a sectional view taken along a direction C-C in FIG.17;

[0037]FIG. 19 shows a printed circuit board mounting an optical part onthe optical-electric printed wiring board 62 according to the thirdembodiment;

[0038]FIGS. 20A to 20E are sectional views showing the individual stepsof a method of fabricating an optical wiring layer 11;

[0039]FIGS. 21A to 21E are sectional views showing the individual stepsof a method of fabricating an electric wiring substrate 12;

[0040]FIGS. 22A to 22E are sectional views showing the individual stepsof a method of stacking the optical wiring layer 11 on the electricprinted wiring board 12;

[0041]FIG. 23 is a top view showing, from the optical part mountingside, an optical-electric printed wiring board 64 according to thefourth embodiment;

[0042]FIG. 24 is a sectional view taken along a direction C-C in FIG.23;

[0043]FIG. 25 is a sectional view showing an optical-electric printedwiring board 64 mounting a laser light emitting element 22;

[0044]FIGS. 26A to 26E are sectional views showing the individual stepsof a method of fabricating an optical wiring layer 11; and

[0045]FIGS. 27A to 27J are sectional views for explaining a method offabricating an electric wiring substrate 12, and a method of stackingthe optical wiring layer 11 on the electric wiring substrate 12 by usingconductive projections.

DETAILED DESCRIPTION OF THE INVENTION

[0046] Four best modes for carrying out the present invention will bedescribed in turn with reference to the accompanying drawings. In thefollowing description, the same reference numerals denote componentshaving substantially the same functions and arrangements, and aduplicate explanation will be made only where necessary.

[0047] First Embodiment

[0048] An important point of an optical-electric printed wiring boardaccording to the first embodiment is the idea that an optical wiringlayer for mounting an optical part (optical element) and a substratehaving electric interconnects are stacked.

[0049]FIG. 1 is a top view showing, from the optical part mounting side,an optical-electric printed wiring board 10 according to the firstembodiment.

[0050]FIG. 2 is a sectional view taken along a direction C-C in FIG. 1.

[0051] As shown in FIGS. 1 and 2, the optical-electric printed wiringboard 10 has a structure in which an optical wiring layer 11 is stackedon a substrate 12.

[0052] First, the arrangement of this optical-electric printed wiringboard 10 will be explained with reference to FIGS. 1 and 2, in the orderof an optical wiring system, an electric wiring system, and connectingmeans for electrically connecting an optical part or the like mounted onthe optical wiring layer 11 and the electric wiring system.

[0053] The optical wiring layer 11 includes a core 111 through which anoptical signal propagates, and a clad 113 confined in this core 111. Thepattern of the core 111 is formed by photolithography as will bedescribed later. The position of the core Ill can be determined byalignment marks (not shown) formed on a support substrate. When therefractive index of the material forming the core 111 is made higherthan that of the clad 113, an optical signal propagates in the core 111.

[0054] As the material of the core 111, it is possible to select a resinhaving a refractive index suited to a wavelength to be guided. Examplesare a fluorine-based polyimide resin and a polymethylmethacrylate resin.

[0055] The material of the clad 113 can be, e.g., a fluorinatedpolyimide-based resin or a fluorinated epoxy-based resin.

[0056] The core 111 has a mirror 115 so formed that the incident angleof an optical signal is 45°. An optical signal propagates between thecore 111 and an optical part (e.g., a laser diode or photodiode) viathis mirror 115 (FIGS. 3 and 4). The mirror 115 can be formed byperforation by etching or a laser, by using a metal mask formed on theoptical wiring layer by photolithography. The position of this mirror115 can be determined by alignment marks (not shown) formed on thesubstrate 12. The interface (a surface away from the core 111) of themirror 115 is brought into contact with a resin having a refractiveindex lower than that of the core 111, or with the air. A thin metalfilm can also be formed on this interface.

[0057] The substrate 12 has electric interconnects 120, 121, 122, and123 on its surface. This substrate 12 can be either a single-layeredinsulating substrate or a multilayered electric wiring substrate. As thematerial of the substrate 12, it is possible to use, e.g., a polyimidefilm, a substrate formed by impregnating a glass cloth with an epoxyresin or the like, or a ceramic substrate.

[0058] In a peripheral portion of the mirror 115, conductive projections14, 15, 16, and 17 are formed to extend through the optical wiring layer11 in the stacking direction.

[0059] These conductive projections 14, 15, 16, and 17 are electricalconnecting means for electrically connecting an optical part (laserlight emitting element or light receiving element) and the electricinterconnects 120, 121, 122, and 123, respectively. An optical part orthe like is directly soldered to end faces 14 a, 15 a, 16 a, and 17 a(FIG. 1), on the side of the optical wiring layer 11, of theseconductive projections 14, 15, 16, and 17, respectively. If necessary,these end faces 14 a, 15 a, 16 a, and 17 a can be surface-treated, e.g.,plated with Ni/Au.

[0060] The optical-electric printed wiring board according to thepresent invention mounts an electric part such as an IC as well as anoptical part. When this is the case, an electric part is electricallyconnected to the electric interconnects 120, 121, 122, and 123 on thesubstrate 12 via the conductive projections 14, 15, 16, and 17,respectively.

[0061] These conductive projections are formed using photolithographyand plating. Accordingly, the formation positions can be determined byalignment marks (not shown) formed on the substrate 12. The number ofconductive projections usually corresponds to the number of connectingterminals 221 of an optical part or the like. However, the number ofconductive projections can also be increased or decreased wherenecessary.

[0062]FIG. 3 is a sectional view in which the leads 221 of a laser lightemitting element 22 such as a semiconductor laser are soldered by solder24 onto the end faces 14 a, 15 a, 16 a, and 17 a of the conductiveprojections. As shown in FIG. 3, a laser beam 31 emitted from a laseremitting surface 220 of the laser light emitting element 22 is reflectedby the mirror 115, and propagates in the core 111.

[0063]FIG. 4 is a sectional view in which leads 231 of a light receivingelement 23 such as a photodiode are soldered by solder 24 onto the endfaces 14 a, 15 a, 16 a, and 17 a of the conductive projections. As shownin FIG. 4, a laser beam 31 propagating in the optical wiring layer 11 isreflected by the mirror 115, and incident on a light receiving surface230 of the light receiving element 23.

[0064] As described above, the relative positional relationship betweenthe optical wiring core 111, the conductive projections 14, 15, 16, and17 (or the end faces 14 a, 15 a, 16 a, and 17 a) for mounting an opticalpart, and the mirror 111 in the optical wiring layer 111 is determinedon the basis of alignment marks (not shown). Therefore, the optical axiscan be aligned with the intended one with extremely high accuracy. Also,the optical axis of an optical part can be optically aligned with thatof the core 111 of the optical wiring layer 11 only by arranging leadsof the optical part in the positions of the end faces 14 a, 15 a, 16 a,and 17 a of the conductive projections. Consequently, the optical partcan be automatically soldered with ease in a reflow furnace or the like.

[0065] Note that conductive projections for soldering an electric partcan also be formed on the optical wiring layer. Note also that electricinterconnects can be additionally formed on the optical wiring layer 11.The arrangement of the conductive projections for an electric part canbe the same as the optical part conductive projections described above.

[0066] If necessary, additional electric interconnects for electricallyconnecting an optical part or electric part and the conductiveprojections can also be formed on the optical wiring layer 11.

[0067] First Fabrication Method of Optical-Electric Printed Wiring Board

[0068] The first fabrication method of the optical-electric printedwiring board according to the present invention will be described below.

[0069] An outline of the optical-electric printed wiring boardfabrication method according to the present invention is as follows.

[0070] First, conductive projections 14, 15, 16, and 17 are formed byphotolithography and plating on a substrate 12 having electricinterconnects 120, 121, 122, and 123.

[0071] On top of the resultant structure, a cladding layer 113 a and acore layer 111 a are formed in this order by coating.

[0072] Next, the core layer 111a is processed into a predetermined shapeby photolithography and etching, thereby forming a core 111 as anoptical interconnect.

[0073] In addition, the core 111 is coated with a cladding layer 113 bto form a clad 113, obtaining an optical wiring layer 11.

[0074] After that, portions of the clad 113 on end faces 14 a, 15 a, 16a, and 17 a of the conductive projections 14, 15, 16, and 17,respectively, are removed to expose these end faces on the surface ofthe optical wiring layer.

[0075] Furthermore, photolithography, plating, and etching are used toform electric interconnects and a metal mask opening for forming amirror 115, and the mirror 115 is formed by dry etching.

[0076] A particularly important point of this fabrication method is thatthe optical wiring layer 11 is formed by directly mounting the clad 113,the core 111, and the like on the electric wiring substrate 12 havingconductive projections. This idea can simplify the fabrication processand improve the fabrication efficiency.

[0077] The fabrication method of the optical-electric printed wiringboard 10 shown in FIG. 2 will be described in more detail below withreference to the accompanying drawings. The explanation will be madewith reference to FIGS. 5A to 50 by particularly focusing attention onthe conductive projections 14, 15, 16, and 17 for mounting an opticalpart, which electrically connect to the electric interconnects 120, 121,122, and 123 on the substrate 12.

[0078]FIGS. 5A to 50 are views showing the individual steps of thefabrication method of an optical-electric printed wiring board 50. FIGS.SA to 50 are arranged in the order of execution.

[0079] First, as shown in FIG. 5A, a metal thin-film layer 13 made of Crand copper is formed by sputtering on a copper polyimide multilayeredsubstrate 12 having a copper wiring layer 18.

[0080] As shown in FIG. 5B, PMER (manufactured by TOKYO OHKA KOGYO CO.,LTD.) is formed to have a thickness of 40 μm as a photoresist 19 by rollcoating. Exposure and development are performed to form openings 20having a diameter of 100 μm.

[0081] Subsequently, the metal thin-film layer 13 is used as a cathodeto plate copper to about the film thickness of the photoresist at roomtemperature in a copper sulfate bath. In addition, 2 μm of Ni and 0.05μm of Au are formed by electroless plating, thereby forming conductiveprojections 14, 15, 16, and 17 (14 and 16 are not shown) as shown inFIG. 5C.

[0082] As shown in FIG. 5D, the photoresist 19 is removed by a dedicatedstripping solution.

[0083] As shown in FIG. 5E, the metal thin-film layer 13 is removed byan etching solution.

[0084] As shown in FIG. 5F, polyimide OPI-N1005 (Hitachi Chemical Co.,Ltd.) is formed by spin coating and imidized at 350° C., as a claddinglayer 113 a. The film thickness is 15 μm.

[0085] As shown in FIG. 5G, polyimide OPI-N1305 (Hitachi Chemical Co.,Ltd.) is formed by spin coating and imidized at 350° C., as a core layer111 a. The film thickness is 8 μm.

[0086] Al is deposited on the surface of the core layer 111 a. apredetermined pattern of a photoresist is formed, and an Al metal maskis formed by an etching solution. Furthermore, oxygen gas is used toetch the core layer 111 a by reactive ion etching, thereby etching awaythe Al film to form a core 111 shown in FIG. 5H.

[0087] As shown in FIG. 5I, OPI-N1005 is formed by coating and imidizedas a cladding layer 113 b on the core 111. The film thickness of thiscladding layer 113 b is 15 μm on the core 111.

[0088] As shown in FIG. 5J, the outermost surface of this substrate isevenly dry-etched to expose end faces 14 a, 15 a, 16 a, and 17 a (14 aand 16 a are not shown) of the conductive projections.

[0089] As shown in FIG. 5K, a thin metal film 30 made of Cr and copperis formed on the dry-etched outermost surface by sputtering.

[0090] As shown in FIG. 5L, PMER is used as a photoresist 31, an openingfor forming an electric interconnect 33 is formed, and the electricinterconnect 33 is formed by electric copper plating.

[0091] The photoresist 31 is removed by a dedicated stripping solution.After that, as shown in FIG. 5M, a 2-μm thick photoresist 34 is againformed by spin coating, and an opening 35 for etching the underlyingthin metal film 30 is formed.

[0092] As shown in FIG. 5N, a pattern 36 for mirror formation is formedin the thin metal film 30 by an etching solution, and the photoresist isremoved by a dedicated stripping solution. After that, the substrate 12is inclined 45°, and perforation is performed by reactive ion etchingusing oxygen gas, thereby forming a mirror 115.

[0093] As shown in FIG. 50, the thin metal film 30 is etched away tocomplete the optical-electric printed wiring board 10 of the presentinvention.

[0094] A printed circuit board can be obtained by fixing an optical part(e.g., a laser or photodiode) and an electric part (e.g., a CPU ormemory) to the optical-electric printed wiring board 10 obtained asdescribed above, particularly to the end faces of the conductiveprojections, by using solder.

[0095] The following effects can be obtained from the above arrangement.

[0096] First, since the optical wiring layer 11 is formed on thesubstrate 12 having the electric interconnects 120, 121, 122, and 123,high-density mounting or miniaturization is possible.

[0097] Second, the relative positional relationship between the patternof the core 111, the end faces 14 a, 15 a, 16 a, and 17 a of theconductive projections for mounting an optical part, and the mirror 115can be matched with the intended one with extremely high accuracy. Thisfacilitates aligning the optical axis of an optical part with that ofthe core 111 as an optical interconnect. As a consequence, an opticalpart and an electric part can be easily mounted.

[0098] Third, when an optical part or an electric part is to besoldered, the part is directly connected to the conductive projections14, 15, 16, and 17 formed by plating. Therefore, the reliability of theconnection can be improved without any influence of solder melting heat.At the same time, the reliability of the connection between eachelectric interconnect on the substrate 12 and the optical or electricpart also improves.

[0099] Fourth, since electric interconnects can also be formed on theoptical wiring layer 11 where necessary, interference between theelectric interconnects can be further suppressed.

[0100] Second Fabrication Method of Optical-Electric Printed WiringBoard

[0101] The second fabrication method of the optical-electric printedwiring board 10 according to the present invention will be explainedbelow.

[0102]FIG. 6 is a plan view of a portion where an optical part is to bemounted on the optical-electric printed wiring board 10 obtained by thesecond fabrication method. FIG. 7A is a sectional view taken along adirection C-C (along a core 111) in FIG. 6.

[0103] A particularly important point of this second fabrication methodis that an optical wiring layer 11 and an electric wiring substrate 12having conductive projections are separately fabricated and adhered viaan adhesive layer, thereby fabricating the optical-electric printedwiring board 10.

[0104] Details of the second fabrication method will be described belowwith reference to the accompanying drawings, in the order of (1) anoptical wiring layer fabrication method, (2) an electric wiringsubstrate fabrication method, and (3) an optical-electric printed wiringboard fabrication method.

[0105] (1) Optical Wiring Layer Fabrication Method

[0106] A film-like optical wiring layer 11 is formed following theprocedure shown in FIGS. 8A to 8E as will be explained below.

[0107] As shown in FIG. 8A, a silicon wafer 40 is coated with a claddinglayer 113 a about 20 to 50 μm thick. If this cladding layer 113 a is apolyamic acid solution, the layer is calcined for 1 to 2 hr at 350° C.for imidization. If the cladding layer 113 a is an epoxy-based resin,the layer is UV-cured or polymerized at 100 to 200° C.

[0108] Next, as shown in FIG. 8B, an 8-μm thick core layer 111 a servingas an optical waveguide is evenly formed by an appropriate coatingmethod by selecting a resin, e.g., fluorine-based polyimide or apolymethylmethacrylate resin, having a refractive index suited to thewavelength to be guided.

[0109] As shown in FIG. 8C, a core 111 is formed from the core layer 111a. Methods of forming this core 11 are classified into, e.g., thefollowing two methods in accordance with whether or not the core layer111 a is photosensitive. That is, if the core layer 111 a isphotosensitive, the core 111 is formed by patterning by the conventionalphotolithography method, and cured in accordance with the material. Ifthe core layer 111 a is not photosensitive, after this core layer 111 ais cured, a metal mask having a predetermined pattern is formed, and thepattern of the core 111 is formed by RIE dry etching.

[0110] After that, a cladding layer 113 b about 20 to 50 μm thick isformed by coating by using the same material as the cladding layer 113 apreviously formed, obtaining a clad 113.

[0111] Subsequently, as shown in FIG. 8D, through holes 41 extendingthrough an optical wiring layer made up of the core 111 and the clad 113are formed in predetermined positions of this optical wiring layer.These holes 41 can be formed by irradiation with an excimer laser via amask having a predetermined pattern.

[0112] Finally, as shown in FIG. 8E, the film-like optical wiring layer11 including the through holes 41 can be formed by peeling this opticalwiring layer 11 from the silicon wafer 40.

[0113] In the above fabrication method, the through holes 41 are formedon the silicon wafer 40. Therefore, these through holes 41 can bereadily formed with high accuracy. Also, the optical wiring layer 11 ispeeled from the silicon wafer 40 after the through holes 41 are formed.Accordingly, tailings produced during the formation of the through holes41 can be completely removed.

[0114] (2) Electric Wiring Substrate Fabrication Method

[0115] The electric wiring substrate fabrication method will bedescribed with reference to FIGS. 9A to 9E.

[0116] As shown in FIG. 9A, on an appropriate insulating substrate 12such as a glass epoxy substrate, a thin copper film about 20 μm thick isformed by, e.g., plating, sputtering, or evaporation. Also, a desiredmetal interconnect 43 is formed by the conventional photolithographymethod. To form conductive projections, a thin metal film 44 is formedby sputtering.

[0117] Next, as shown in FIG. 9B, the thin metal film 44 is coated witha resist 45, and this resist 45 is developed to form openings 46.

[0118] As shown in FIG. 9C, the thin metal film 44 is used as a cathodeto perform copper plating, thereby filling the openings 46 with copperas much as possible.

[0119] As shown in FIG. 9D, the resist 45 is removed.

[0120] Finally, as shown in FIG. 9E, the thin metal film 44 is etchedaway. Consequently, electric interconnects 120, 121, 122, and 123 andconductive projections 14, 15, 16, and 17 on these electricinterconnects can be formed (the electric interconnects 120 and 122 andthe conductive projections 14 and 16 are not shown).

[0121] The shape of the through hole 41 which allows each conductiveprojection to extend through it can be selected from a column, a squarepillar, or the like, in accordance with the shape of the mask. Theheight of this through hole 41 can be controlled by the film thicknessof the resist 45 or the plating time. According to the experimentconducted by the present inventors, both the diameter and height arepreferably about 50 to 100 μm.

[0122] (3) Optical-Electric Printed Wiring Board Fabrication Method

[0123] The method of stacking the optical wiring layer 11 on theelectric wiring substrate 12 by using the conductive projections will beexplained below with reference to FIGS. 10A to 10E.

[0124] First, referring to FIG. 10A, the conductive projections 14, 15,16, and 17 are used as alignment guides when the optical wiring layer 11and the electric substrate 12 are stacked. That is, the optical wiringlayer 11 and the electric substrate 12 are so stacked that theconductive projections made of a conductive metal or the like extendthrough the through holes 41 in the optical wiring layer 11. It isdesirable that the optical wiring layer 11 and the electric substrate 12be completely adhered by forming an adhesive layer 47 by coating on thatside of the optical wiring layer 11, which comes in contact with theelectric substrate 12. Accordingly, the conductive projections 14, 15,16, and 17 have the function of support guides for the upper and lowersubstrates (the optical wiring layer 11 and the electric wiringsubstrate 12), and the function of electrically connecting the electricinterconnects 120, 121, 122, and 123 and an optical part or the likemounted on the optical wiring layer 11.

[0125] Next, as shown in FIG. 10B, a thin metal film 30 is formed bysputtering on the surface of the stacked optical wiring layer 11.

[0126] After this thin metal film 30 is coated with a photoresist 31, asshown in FIG. 10C, exposure and development are performed to form aphotoresist opening 35 for mirror formation.

[0127] An opening 36 is then formed in the thin metal film 30 byetching, and a metal mask for mirror formation is formed. In addition,as shown in FIG. 10D, the substrate is inclined 45°, and a mirror 115 isformed by RIE dry etching.

[0128] Finally, as shown in FIG. 10E, the metal mask is dissolved awayto complete the optical-electric printed wiring board of the presentinvention.

[0129] In this second fabrication method, the optical wiring layer 11and the electric wiring substrate 12 having the conductive projectionsare separately fabricated. Therefore, the mirror 115 having a shapeshown in FIG. 7C can also be formed. That is, when the optical wiringlayer 11 is to be independently formed, the mirror 115 having the shapeshown in FIG. 7C can be formed by etching, a dicing saw, or the like byfixing the optical wiring layer 11 to a predetermined support after,e.g., the step shown in FIG. 8E.

[0130] (4) Printed Circuit Board Fabrication Method

[0131] A printed circuit board can be obtained by fixing an optical part(e.g., a laser or photodiode) and an electric part (e.g., a CPU ormemory) to the optical-electric printed wiring board 10 obtained asdescribed above, particularly to the end faces of the conductiveprojections.

[0132]FIG. 7B is a view showing a printed circuit board 9 fabricated bysoldering terminals 221 of an optical part 22 containing a lightemitting laser to the conductive projections 14, 15, 16, and 17 bysolder 24.

[0133] In this printed circuit board 9 shown in FIG. 7B, the end faces14 a, 15 a, 16 a, and 17 a (FIG. 6) of the conductive projections 14,15, 16, and 17, respectively, are used as pads. As described previously,the conductive projections 14, 15, 16, and 17 are used to obtainelectrical conduction between an optical part such as a semiconductorlaser and the electric interconnects 120, 121, 122, and 123, and ensurealignment with the mirror 115. Accordingly, the reliability of theconnection concerning optical waveguiding between the optical part andthe optical wiring layer can be improved. This is so because the use ofthe end face of each conductive projection as a pad can assure highlyaccurate alignment, and the self-alignment effect can be obtained bysolder connection.

[0134] As another desirable arrangement, metal pads conducting to theconductive projections 14, 15, 16, and 17 can also be formed on theoptical wiring layer 11 by using photolithography.

[0135] In practice, the width of the optical wiring layer 11 and themirror 115 is a few μm. Therefore, a laser beam must be reliablyconverged to this range. For this purpose, the end faces 14 a, 15 a, 16a, and 17 a as metal pads to be connected to the terminals 221 of anoptical part can also be recessed from their peripheral portion.

[0136] Furthermore, guides for guiding the connecting positions of theterminals 221 of an optical part can be formed around the end faces 14a, 15 a, 16 a, and 17 a. These guides can be either conductive orinsulating and are so formed as to surround the conductive projections14, 15, 16, and 17 (i.e., the end faces 14 a, 15 a, 16 a, and 17 a).

[0137] These means for improving the accuracy of alignment between alaser beam and the mirror will be explained in detail later as thesecond to fourth embodiments.

[0138] Accordingly, the following effects can be obtained by theabove-mentioned arrangement.

[0139] First, an optical wiring layer is formed on a substrate havingelectric interconnects, so high-density mounting or miniaturization ispossible.

[0140] Second, the relative positional relationship between the patternof the core 111, the end faces 14 a, 15 a, 16 a, and 17 a of theconductive projections for mounting an optical part, and the mirror 115can be matched with the intended one with extremely high accuracy. Thisfacilitates aligning the optical axis of an optical part with that ofthe optical wiring layer 11, without any alignment step. Furthermore, anoptical part and an electric part can be automatically mounted at thesame time.

[0141] Third, when an optical part or an electric part is to besoldered, the part is directly connected to the conductive projectionsformed by plating. Therefore, the reliability of the connection can beimproved without any influence of solder melting heat. At the same time,the reliability of the connection between the electric interconnects120, 121, 122, and 123 on the substrate 12 and the optical or electricpart also improves.

[0142] Fourth, since electric interconnects can also be formed on theoptical wiring layer 11 where necessary, interference between theelectric interconnects can be further suppressed.

[0143] Fifth, the optical wiring layer 11 and the electric substrate 12can be fabricated independently of each other. Furthermore, the opticalwiring layer 11 can be easily and accurately stacked on the electricsubstrate 12 via the conductive projections as guides.

[0144] Sixth, in the fabrication process of the optical-electric printedcircuit board 10, no tailings stay in the openings 41 for formingconductive projections. Hence, the optical wiring layer 11 and theelectric wiring substrate 12 can be electrically reliably connected.

[0145] More specifically, if the openings 41 for forming conductiveprojections are formed by laser beam irradiation or plating, thefollowing problems arise. First, tailings produced by thermaldestruction of the resin component of the support of the electric wiringsubstrate stick to the metal film on the bottom of each opening 41 or tothe circumferential surface of the opening 41. These tailings cannot becompletely removed even through various cleaning steps. Second, theunremoved tailings interfere with electrical conduction between theconductive projection and the underlying thin metal film via the thincopper film covering the circumferential surface of the opening 41.Third, the adhesion of the thin copper-plated film of the conductiveprojection to the underlying optical wiring layer film is low. When anoptical part is to be soldered, therefore, the thin copper-plated filmreadily peels upon heating. However, the above arrangement does not poseany of these problems.

[0146] The two methods of fabricating the optical-electric printedcircuit board have been described above. By either of these two methods,optical printed wiring boards and printed circuit boards according tothe second, third, and fourth embodiments to be described below can befabricated.

[0147] Note that for the sake of brevity, examples obtained by only thesecond fabrication method described above are explained in the second,third, and fourth embodiments.

[0148] Second Embodiment

[0149] An optical-electric printed wiring board 60 according to thesecond embodiment will be described below.

[0150] An important point of the optical-electric printed wiring boardaccording to this second embodiment is that guides for guiding theconnecting positions of terminals 221 of an optical part is formedaround end faces 14 a, 15 a, 16 a, and 17 a. With these guides, a laserbeam emitted from or incident on an optical part is reliably convergedto a desired range. As a consequence, the optical axis of a lightemitting laser or the like and that of a mirror 115 having a width of afew μm can be accurately aligned. In addition, the optical part can bestrongly fixed and, if necessary, can be electrically connected toelectric interconnects on an electric wiring substrate 12.

[0151]FIG. 11 is a top view showing, from the optical part mountingside, the optical-electric printed wiring board 60 according to thesecond embodiment.

[0152]FIG. 12A is a sectional view taken along a direction C-C in FIG.11.

[0153] As shown in FIGS. 11 and 12A, this optical-electric printedwiring board 60 has a structure in which guide portions 48 are formed onan optical wiring layer 11 of an optical-electric printed circuit board10 described in the first embodiment. These guide portions 48 are formedby forming, e.g., recesses 51 and 53 in that surface of the opticalwiring layer 11, which mounts an optical part or the like. Note thatthese recesses correspond to conductive projections. Although not shown,therefore, the optical-electric printed wiring board 60 has recessesother than these recesses 51 and 53.

[0154] Each recess is so formed that a terminal of an optical part canbe smoothly accommodated, and desirably, soldered. When the shapes ofthe recess and a terminal of an optical part are similar to each other,the range within which the terminal is movable is restricted, so theobject can be achieved more favorably. Note that conductive projectionsaccording to this embodiment are also formed by photolithography andplating. Therefore, the positions of these conductive projections can bedetermined by alignment marks (not shown) formed on the substrate 12.

[0155] Furthermore, the circumferential surface and upper peripheralportion of each recess can be surface-treated, e.g., plated with Ni/Au.

[0156]FIGS. 12B and 12C are enlarged views showing the recess 51 and itsvicinity. FIG. 12C particularly depicts an example in which thecircumferential surface and peripheral portion of the recess 51 aresubjected to a surface treatment, e.g., Ni/Au plating. FIG. 13 shows anexample in which a laser light emitting element 22 is mounted on theoptical printed wiring board 60.

[0157] As shown in FIG. 13, the tip of the terminal 221 of the laserlight emitting element 22 is accommodated in a corresponding recess, andbrought into contact with and soldered to the end face of acorresponding conductive projection. This can further improve theaccuracy of alignment between the board 60 and the optical part or thelike.

[0158] As especially shown in FIG. 12C, when the circumferential surfaceand vicinity of each recess are subjected to a surface treatment such asNi/Au plating, the area for an electrical contact can be widened, andthis can improve the reliability of the device.

[0159] Fabrication Method of Optical-electric Printed Wiring Board

[0160] The fabrication method of the optical-electric printed wiringboard 60 according to the second embodiment will be described in detailbelow with reference to the accompanying drawings, in the order of (1)an optical wiring layer fabrication method, (2) an electric wiringsubstrate fabrication method, (3) an optical-electric printed wiringboard fabrication method, and (4) a printed circuit board fabricationmethod.

[0161] (1) Optical Wiring Layer Fabrication Method

[0162]FIGS. 14A to 14E are sectional views showing the individual stepsof the method of fabricating the optical-electric printed wiring board60. FIGS. 14A to 14E are arranged in the order of execution.

[0163] As shown in FIG. 14A, a silicon wafer 40 is coated with acladding layer 113 a (a support medium of an optical wiring layer forguiding light, e.g., fluorinated polyamic acid as a precursor of afluorinated polyimide-based resin or a fluorinated epoxy-based resin isused) about 20 to 100 μm thick. If this cladding layer 113 a is apolyamic acid solution, the layer is calcined for 1 to 2 hr at 350° C.for imidization. If the cladding layer 113 a is an epoxy-based resin,the layer is UV-cured or polymerized at 100 to 200° C.

[0164] Next, as shown in FIG. 14B, an 8-μm thick core layer 111 aserving as an optical waveguide is evenly formed by an appropriatecoating method by selecting a resin, e.g., a fluorine-based polyamicacid solution or a polymethylmethacrylate resin solution, having arefractive index suited to the wavelength to be guided

[0165] As shown in FIG. 14C, if the core layer 111 a is photosensitive,a core 111 is formed by patterning by the conventional photolithographymethod, and cured in accordance with the material. If the core layer 111a is not photosensitive, after this core layer 111 a is cured, a metalmask having a predetermined pattern is formed, and a waveguide patternis formed by RIE dry etching. In addition, a layer about 20 to 100 μmthick is formed by coating by using the same material as the claddinglayer previously formed.

[0166] Subsequently, as shown in FIG. 14D, through holes 41 are formedin predetermined positions of the optical wiring layer. That is, theseholes are formed by irradiation with an excimer laser via a mask havinga predetermined pattern.

[0167] Finally, as shown in FIG. 14E, a film-like optical wiring layer11 including the through holes 41 can be formed by peeling this opticalwiring layer 11 from the silicon wafer 40. In this method, perfectthrough holes could be formed, and no tailings remained.

[0168] (2) Electric Wiring Substrate Fabrication Method

[0169] The method of fabricating the electric wiring substrate 12 willbe described with reference to FIGS. 15A to 15E.

[0170] As shown in FIG. ISA, on an appropriate insulating substrate 12such as a glass epoxy substrate, a thin copper film about 20 μm thick isformed by, e.g., plating, sputtering, or evaporation. Also, a desiredmetal interconnect 43 (i.e., electric interconnects 120, 121, 122, and123) is formed by the conventional photolithography method. To formconductive projections, a thin metal film 44 is formed by sputtering.

[0171] Subsequently, as shown in FIG. 15B, the thin metal film 44 iscoated with a resist 45, and this resist 45 is developed to formopenings 46.

[0172] As shown in FIG. 15C, the thin metal film 44 is used as a cathodeto perform copper plating, thereby filling the openings 46 with copperas much as possible.

[0173] As shown in FIG. 15D, the resist 45 is removed.

[0174] Finally, as shown in FIG. 15E, the thin metal film 44 is etchedaway to form conductive projections 14, 15, 16, and 17 (the conductiveprojections 14 and 16 are not shown) on the electric interconnect.Consequently, the electric wiring substrate 12 can be obtained.

[0175] The conductive projections are desirably formed using a maskhaving a shape, such as a column or a square pillar, matching the shapeof a terminal of an optical part. The height of this conductiveprojection can be controlled by the film thickness of the resist or theplating time. According to the experiment conducted by the presentinventors, the diameter and height of the conductive projection arepreferably about 50 to 100 μm and about 20 to 200 μm, respectively.

[0176] (3) Optical-Electric Printed Wiring Board Fabrication Method

[0177] The method of stacking the optical wiring layer 11 on theelectric wiring substrate 12 by using the conductive projections will beexplained below with reference to FIGS. 16A to 16E.

[0178] First, referring to FIG. 16A, a plurality of conductiveprojections are used as guides for stacking the optical wiring layer 11on the electric substrate 12 by aligning these layer and substrate.

[0179] That is, the optical wiring layer 11 and the electric substrate12 are so stacked that columns made of a conductive metal or the likeextend to the middle of the film through the through holes 46 in theoptical wiring layer 11. It is desirable that the optical wiring layer11 and the electric substrate 12 be completely adhered by forming anadhesive layer 47 by coating on that side of the optical wiring layer11, which comes in contact with the electric substrate 12. As shown inFIG. 16A, one or both of the film thickness of the optical wiring layer11 and the height of the metal guide are adjusted such that the depth ofthe recess from the film surface is large enough to accommodate aterminal of an optical part, and desirably, 20 to 200 μm.

[0180] Next, as shown in FIG. 16B, a thin metal film 30 is formed bysputtering on the surface of the stacked optical wiring layer 11.

[0181] As shown in FIG. 16C, a photoresist 31 is formed by coating.Exposure and development are then performed to form a photoresistopening 35 for mirror formation.

[0182] An opening 36 is formed in the thin metal film 30 by etching, anda metal mask for mirror formation is formed. In addition, as shown inFIG. 16D, the substrate is inclined 45°, and a mirror 115 is formed byRIE dry etching.

[0183] Finally, the metal mask is dissolved away to complete theoptical-electric printed wiring board 60 as shown in FIG. 16E.

[0184] Another method of fabricating conductive projections is toperform laser beam irradiation or dry etching, from the optical wiringlayer side, for appropriate positions on the optical-electric printedwiring board. The electric interconnect formed on the substrate 12functions as a stopper, so the opening 36 (via hole) can be formed tothis depth. Plating is subsequently performed to fill this opening 36with a metal. When this plating is stopped in the middle of the opticalwiring layer 11 by adjusting the plating time, a recess 51 having adesired depth can be formed. By this method, a recess can be formed deepenough to reach the electric wiring layer.

[0185] (4) Printed Circuit Board Fabrication Method

[0186] The method of fabricating a printed circuit board obtained bymounting an optical part on the optical-electric printed wiring board 60according to the second embodiment will be described below (withreference to FIG. 13).

[0187] First, a solder ball is placed in a recess 50 and the like in theoptical-electric printed wiring board, and a terminal 221 of an electricpart (a laser or photodiode) is lightly inserted into the recess 50.Assume that the shape of the recesses 50, 51, and the like is a circle80 μm in radius, and the depth of each recess is 50 μm. Note also thatthe number of conducting terminals 221 of an optical part is four, andthe shape of each terminal is a circle 75 μm in radius. Terminals for anelectric part (a CPU or memory) are placed on slightly soldered metalpads.

[0188] According to the experiment conducted by the present inventors,when the device was left to stand in a reflow furnace at a temperatureof 250° for 10 seconds and then cooled, the terminals of an optical partwere fixed in equilibrium positions determined by the shape of therecess and the surface tension of the molten solder, and the opticalaxis of the laser fell within the range of the central position of themirror ±3 μm. When the terminals of an optical part were placed on flatmetal pads having no recess similar to an electric part, the fixedpositions of the optical part were unstable, and errors of about ±50 μmwere produced. The recesses ensured highly accurate alignment betweenthe electrical conduction and the mirror for optical wiring. Inaddition, the apexes of the conductive projections were directlyelectrically connected as pads to the optical part. Consequently, thereliability of the connection also improved.

[0189] Accordingly, the following effects can be obtained by theabove-mentioned arrangement.

[0190] First, the optical wiring layer 11 is formed on the substrate 12having electric interconnects, so high-density mounting orminiaturization is possible.

[0191] Second, the relative positional relationship between the patternof the core 111 including the mirror 115 in the optical wiring layer 11and the individual conductive projections for mounting an optical partcan be matched with the intended one with extremely high accuracy.

[0192] Third, the terminal 221 of an optical part is accuratelyaccommodated in each recess on the optical wiring layer 11. Thisfacilitates optically aligning the optical axis of the optical part withthat of the optical wiring layer 11. Therefore, an optical part and anelectric part can be automatically mounted at the same time.

[0193] Fourth, when an optical part or an electric part is to besoldered, the part is directly connected to conductive projectionsformed by plating. Therefore, the reliability of the connection can beimproved without any influence of solder melting heat. At the same time,the reliability of the connection between the electric interconnects onthe substrate 12 and the optical or electric part also improves.

[0194] Third Embodiment

[0195] An optical-electric printed wiring board 62 according to thethird embodiment will be described below.

[0196] An important point of the optical-electric printed wiring board62 according to this third embodiment is the idea that end faces 14 a,15 a, 16 a, and 17 a of conductive projections project to apredetermined height from the mounting surface of an optical wiringlayer 11, and the projected ends of these conductive projections areconnected to terminals of an optical part or the like (the shape of eachterminal is preferably a recess, and more preferably, similar to the endfaces 14 a, 15 a, 16 a, and 17 a).

[0197] When an optical part or the like is thus connected to theconductive projections, a laser beam emitted from or incident on theoptical part is reliably converged to a desired range. As a consequence,as in the second embodiment, the optical axis of a light emitting laseror the like and that of a mirror 115 having a width of a few μm can beaccurately aligned. In addition, the optical part can be strongly fixedand, if necessary, can be electrically connected to electricinterconnects on an electric wiring substrate 12.

[0198]FIG. 17 is a top view showing, from the optical part mountingside, an example of the optical-electric printed wiring board 62according to the third embodiment.

[0199]FIG. 18 is a sectional view taken along a direction C-C in FIG.17.

[0200] As shown in FIGS. 17 and 18, this optical-electric printed wiringboard 62 has a structure in which, in addition to the structure of theoptical-electric printed wiring board 10 described in the firstembodiment, conductive projections 14, 15, 16, and 17 project to apredetermined height from the mounting surface of the optical wiringlayer 11.

[0201] These conductive projections 14, 15, 16, and 17 are preferably ametal such as copper, because in this case they can conduct to electricinterconnects 120, 121, 122, and 123 formed on the substrate 12.However, these conductive projections need not conduct to the electricinterconnects and can also be simple struts.

[0202] As shown in FIG. 19, the conductive projections 14, 15, 16, and17 projecting from the optical wiring layer 11 are smoothly accommodatedin recesses in terminals 223 of an optical part 22, and the end portionsof these conductive projections are soldered to the recesses of theoptical part. Especially when the recess in the terminal 223 of theoptical part and the end portion of each conductive projection havesimilar shapes and engage with each other, the range within which theoptical part is movable is restricted. Accordingly, the optical axis ofthe light emitting laser 22 and that of the mirror 115 having a width ofa few μm can be accurately aligned, and the light emitting laser 22 isstrongly fixed. Also, electric interconnects additionally formed on theoptical wiring substrate 11 can be electrically connected to theelectric interconnects 14, 15, 16, and 17 where necessary.

[0203] The recess in the terminal 223 of the optical part 22 can beaccurately formed by plating by using any conventional method such asbump formation. Furthermore, the reliability of the electricalconnection can be further improved by performing a surface treatment,e.g., Ni/Au plating, for the end portions of the conductive projections14, 15, 16, and 17.

[0204] Although not shown in FIG. 19, an electric part such as an IC isformed, if necessary, on the optical wiring layer 11 (or on theconductive projections) similar to an optical part, and electricallyconnected to the electric interconnects 120, 121, 122, and 123 via theconductive projections. Since these conductive projections are formed byphotolithography and plating, their positions can be determined byalignment marks (not shown) formed on the substrate 12.

[0205] Fabrication Method of Optical-electric Printed Wiring Board

[0206] The fabrication method of the optical-electric printed wiringboard 62 according to the third embodiment will be described in detailbelow with reference to the accompanying drawings, in the order of (1)an optical wiring layer fabrication method, (2) an electric wiringsubstrate fabrication method, (3) an optical-electric printed wiringboard fabrication method, and (4) a printed circuit board fabricationmethod.

[0207] (1) Optical Wiring Layer Fabrication Method

[0208]FIGS. 20A to 20E are sectional views showing the individual stepsof the method of fabricating the optical-electric printed wiring board62. FIGS. 20A to 20E are arranged in the order of execution.

[0209] As shown in FIG. 20A, a silicon wafer 40 is coated with acladding layer 113 a (a support medium of an optical wiring layer forguiding light, e.g., fluorinated polyamic acid as a precursor of afluorinated polyimide-based resin or a fluorinated epoxy-based resin isused) about 20 to 100 μm thick. If this cladding layer 113 a is apolyamic acid solution, the layer is calcined for 1 to 2 hr at 350° C.for imidization. If the cladding layer 113 a is an epoxy-based resin,the layer is UV-cured or polymerized at 100 to 200° C.

[0210] Next, as shown in FIG. 20B, an 8-μm thick core layer 111 aserving as an optical waveguide is evenly formed by an appropriatecoating method by selecting a resin, e.g., a fluorine-based polyamicacid solution or a polymethylmethacrylate resin solution, having arefractive index suited to the wavelength to be guided.

[0211] As shown in FIG. 20C, if the core layer 111 a is photosensitive,a core 111 is formed by patterning by the conventional photolithographymethod, and cured in accordance with the material. If the core layer 111a is not photosensitive, after this core layer 111 a is cured, a metalmask having a predetermined pattern is formed, and a waveguide patternis formed by RIE dry etching.

[0212] In addition, as shown in FIG. 20D, a portion of the claddinglayer 113 a previously formed and the core 111 are coated with a layer113 b about 20 to 100 μm thick made of the same material as the claddinglayer 113 a. Through holes 41 are then formed in predeterminedpositions. These holes 41 are formed by irradiation with an excimerlaser via a mask having a predetermined pattern.

[0213] Finally, as shown in FIG. 20E, a film-like optical wiring layer11 including the through holes 41 can be formed by peeling this opticalwiring layer 11 from the silicon wafer 40.

[0214] In this method, perfect through holes 41 can be formed with notailings left behind.

[0215] (2) Electric Wiring Substrate Fabrication Method

[0216] The method of fabricating the electric wiring substrate 12 willbe described with reference to FIGS. 21A to 21E.

[0217] First, as shown in FIG. 21A, on an appropriate insulatingsubstrate 12 such as a glass epoxy substrate, a thin copper film about20 μm thick is formed by, e.g., plating, sputtering, or evaporation.Also, a desired metal interconnect 43 (i.e., electric interconnects 120,121, 122, and 123) is formed by the conventional photolithographymethod. In addition, to form a plurality of struts (conductiveprojections), a thin metal film 44 is formed by sputtering.

[0218] Subsequently, as shown in FIG. 21B, a dry film resist 45 isadhered to the thin metal film 44, and exposure and development areperformed following the conventional procedures to form openings 46.

[0219] As shown in FIG. 21C, the thin metal film 44 is used as a cathodeto perform copper plating, thereby filling the openings 46 with copperas much as possible.

[0220] As shown in FIG. 21D, the resist 45 is removed.

[0221] Finally, as shown in FIG. 21E, the thin metal film 44 is etchedaway to form conductive projections 14, 15, 16, and 17 (the conductiveprojections 14 and 16 are not shown) on the electric interconnect.

[0222] As in the second embodiment, the conductive projections aredesirably formed using a mask having a shape, such as a column or asquare pillar, matching the shape of a terminal of an optical part. Theheight of this conductive projection can be controlled by the filmthickness of the resist or the plating time. According to the experimentconducted by the present inventors, the diameter and height of theconductive projection are preferably about 50 to 500 μm and about 20 to200 μm, respectively.

[0223] (3) Optical-electric Printed Wiring Board Fabrication Method

[0224] The method of stacking the optical wiring layer 11 on theelectric wiring substrate 12 by using the conductive projections will beexplained below with reference to FIGS. 22A to 22E.

[0225] First, referring to FIG. 22A, the conductive projections are usedfor alignment when the optical wiring layer 11 is stacked on theelectric substrate 12. That is, the optical wiring layer 11 and theelectric substrate 12 are so stacked that the conductive projectionsmade of a conductive metal or the like extend through the through holes41 formed in the optical wiring layer 11. The optical wiring layer 11and the electric substrate 12 are desirably completely adhered byforming an adhesive layer 47 by coating.

[0226] Next, as shown in FIG. 22B, a thin metal film 30 is formed bysputtering on the surface of the stacked optical wiring layer 11.

[0227] As shown in FIG. 22C, a photoresist 31 is formed by coating.Exposure and development are then performed to form a photoresistopening 35 for mirror formation.

[0228] As shown in FIG. 22D, an opening 36 is formed in the thin metalfilm 30 by etching, and a metal mask for mirror formation is formed. Inaddition, the substrate is inclined 45°, and a mirror 115 is formed byRIE dry etching.

[0229] Finally, as shown in FIG. 22E, the metal mask is dissolved awayto complete the optical-electric printed wiring board 62 of the presentinvention.

[0230] Another fabrication method of the optical-electric printed wiringboard is a buildup method (the first embodiment). This method is not themethod explained in the third embodiment in which the optical wiringlayer 11 and the electric wiring substrate 12 are stacked after beingseparately fabricated. That is, this method is an optical-electricprinted wiring board fabrication method by which an optical substratematerial is stacked while being patterned directly on the electricwiring substrate 12. As already described previously, when theoptical-electric printed wiring board 62 is to be fabricated by thisbuildup method, the conductive projections can be formed on the electricsubstrate before the optical wiring substrate is built up, or can beformed by forming via holes and filling these holes with plating afterthe optical substrate is stacked.

[0231] (4) Printed Circuit Board Fabrication Method

[0232] The method of fabricating a printed circuit board obtained bymounting an optical part on the optical-electric printed wiring board 62according to the third embodiment will be described below (withreference to FIG. 19).

[0233] First, the recess 24 in each terminal 223 is slightly soldered,and the end portion of each conductive projection is lightly insertedinto the recess of the terminal 223. Assume that the shape of the recessin the terminal 223 is a circle 80 μm in radius, and the depth of therecess is 50 μm. Note also that the number of conductive projections isfour, and the shape of each projection is a circle 75 μm in radius. Whenterminals for an electric part (a CPU or memory) are additionallyformed, this electric part is placed on slightly soldered metal pads.

[0234] According to the experiment conducted by the present inventors,when the device was left to stand in a reflow furnace at a temperatureof 250° for 10 seconds and then cooled, the terminals of an optical partwere fixed in equilibrium positions determined by the shape of therecess and the surface tension of the molten solder, and the opticalaxis of the laser fell within the range of the central position of themirror ±μm. When flat metal terminals having no recess similar to anelectric part and conducting struts were adhered via solder, the fixedpositions of the optical part were unstable, and errors of about ±50 μmwere produced. The recesses ensured highly accurate alignment betweenthe electrical conduction and the mirror for optical wiring. Inaddition, the apexes of the struts were directly electrically connectedas pads to metal terminals of an optical part. Accordingly, thereliability of the electrical connection can be improved.

[0235] The following effects can be obtained by the above-mentionedarrangement.

[0236] First, the optical wiring layer 11 is formed on the substrate 12having electric interconnects, so high-density mounting orminiaturization is possible.

[0237] Second, the relative positional relationship between the patternof the core 111 including the mirror 115 in the optical wiring layer 11and the individual conductive projections for mounting an optical partcan be matched with the intended one with extremely high accuracy.

[0238] Third, the end portion of each conductive projection isaccurately accommodated in the recess of a terminal of an optical part.This facilitates optically aligning the optical axis of the optical partwith that of the optical interconnect. Therefore, an optical part and anelectric part can be automatically mounted at the same time.

[0239] Fourth, when an optical part or an electric part is to besoldered, the part is directly connected to conductive struts formed byplating. Therefore, the reliability of the connection improves withoutany influence of solder melting heat. At the same time, the reliabilityof the connection with the electric interconnects on the substrate alsoimproves.

[0240] Fourth Embodiment

[0241] An optical-electric printed wiring board 64 according to thefourth embodiment will be described below. An important point of theoptical-electric printed wiring board according to this fourthembodiment is that fine projecting guides for guiding the connectingpositions of terminals 221 of an optical part are formed around endfaces 14 a, 15 a, 16 a, and 17 a.

[0242]FIG. 23 is a top view showing, from the optical part mountingside, the optical-electric printed wiring board 62 according to thefourth embodiment. FIG. 24 is a sectional view taken along a directionC-C in FIG. 23. FIG. 25 is a view showing the optical-electric printedwiring board 62 mounting a laser light emitting element 22.

[0243] Referring to FIGS. 23 and 24, a partition wall 53 formed aroundeach of the end faces 14 a, 15 a, 16 a, and 17 a is depicted as anexample of a projecting guide 53.

[0244] As shown in FIG. 25, the terminal 221 of an optical part issmoothly accommodated in a recess partitioned by the partition wall 53,and desirably, soldered to the end face of a conductive projection inthis recess. When the shape of the recess formed by the partition wall53 and the shape of the terminal of an optical part are similar, therange within which the terminal 221 is movable is restricted. Since themovable range of the optical part is regulated, therefore, the opticalaxis of the light emitting laser 22 can be accurately aligned with thatof a mirror 115 having a width of a few μm. In addition, the lightemitting laser 22 can be strongly fixed and, if necessary, can beelectrically connected to electric interconnects.

[0245] This projecting partition wall 53 is desirably capable of tightlyaccommodating the terminal 221 of an optical part. However, thepartition wall 53 need not be continuous like a bank but can bedisconnected in pieces. The partition wall 53 has conductivity toenhance the electrical connection between an optical part and each ofconductive projections 14, 15, 16, and 17. If, however, the partitionwall 53 is used only to fix the position of an optical part, thepartition wall 53 need not conduct to the conductive projection. In thiscase, the positions of these partition walls 53 need not correspond tothe conductive projections 14, 15, 16, and 17.

[0246] Furthermore, an electric part other than an optical part is alsoelectrically connected to electric interconnects on a substrate 12 viathe conductive projections. When this electric part is to be mounted, noprecise alignment is necessary, so the projecting partition walls 53need not be formed. However, it is also possible to form these partitionwalls 53, if necessary, in order to ensure the area of electricalconnection. The projecting partition walls 53 and the conductiveprojections are formed by photolithography and plating. Therefore, thepositions of these walls and projections can be determined, as needed,by alignment marks (not shown) formed on the substrate 12.

[0247] Fabrication Method of Optical-Electric Printed Wiring Board

[0248] The fabrication method of the optical-electric printed wiringboard 64 according to the fourth embodiment will be described in detailbelow with reference to the accompanying drawings, in the order of (1)an optical wiring layer fabrication method, (2) an electric wiringsubstrate fabrication method, and (3) an optical-electric printed wiringboard fabrication method.

[0249] (1) Optical Wiring Layer Fabrication Method

[0250]FIGS. 26A to 26E are sectional views showing the individual stepsof the method of fabricating the optical-electric printed wiring board64. FIGS. 26A to 26E are arranged in the order of execution.

[0251] As shown in FIG. 26A, a silicon wafer 40 is coated with acladding layer 113 a (a support medium of an optical wiring layer forguiding light, e.g., fluorinated polyamic acid as a precursor of afluorinated polyimide-based resin or a fluorinated epoxy-based resin isused) about 20 to 100 μm thick. If this cladding layer 113 a is apolyamic acid solution, the layer is calcined for 1 to 2 hr at 350° C.for imidization. If the cladding layer 113 a is an epoxy-based resin,the layer is UV-cured or polymerized at 100 to 200° C.

[0252] Next, as shown in FIG. 26B, an 8-μm thick core layer 111 aserving as an optical waveguide is evenly formed by an appropriatecoating method by selecting a resin, e.g., a fluorine-based polyamicacid solution or a polymethylmethacrylate resin solution, having arefractive index suited to a wavelength to be guided.

[0253] As shown in FIG. 26C, if the core layer 111 a is photosensitive,an optical waveguide (i.e., a core 111) is formed by patterning by theconventional photolithography method, and cured in accordance with thematerial. If the core layer 111 a is not photosensitive, after this corelayer 111 a is cured, a metal mask having a predetermined pattern isformed, and a waveguide pattern is formed by RIE dry etching. Inaddition, a layer about 20 to 100 μm thick is formed by coating by usingthe same material as the cladding layer previously formed.

[0254] Subsequently, as shown in FIG. 26D, through holes 41 are formedin predetermined positions. That is, these holes are formed byirradiation with an excimer laser via a mask having a predeterminedpattern.

[0255] Finally, as shown in FIG. 26E, a film-like optical wiring layer11 including the through holes 41 can be formed by peeling this opticalwiring layer 11 from the silicon wafer 40.

[0256] In this method, perfect through holes can be formed with notailings left behind.

[0257] (2) Electric Wiring Substrate Fabrication Method

[0258] The method of fabricating the electric wiring substrate 12 willbe described with reference to FIGS. 27A to 27E.

[0259] First, as shown in FIG. 27A, on an appropriate insulatingsubstrate 12 such as a glass epoxy substrate, a thin copper film about20 μm thick is formed by, e.g., plating, sputtering, or evaporation.Also, a desired metal interconnect 43 (i.e., electric interconnects 120,121, 122, and 123) is formed by the conventional photolithographymethod. In addition, to form a plurality of conductive projections, athin metal film 44 is formed by sputtering.

[0260] Next, as shown in FIG. 27B, the thin metal film 44 is coated witha resist 45, and this resist 45 is developed to form openings 44.

[0261] The thin metal film 44 is used as a cathode to perform copperplating, thereby filling the openings 46 with copper as much aspossible.

[0262] As shown in FIG. 27C, the resist 45 is removed.

[0263] Finally, as shown in FIG. 27E, the thin metal film 44 is etchedaway to form conductive projections 14, 15, 16, and 17 on the electricinterconnect 43.

[0264] As already described previously, the conductive projections aredesirably formed using a mask having a shape, such as a column or asquare pillar, matching the shape of a terminal of an optical part. Theheight of this conductive projection can be controlled by the filmthickness of the resist or the plating time. According to the experimentconducted by the present inventors, the diameter and height of theconductive projection are preferably about 50 to 100 μm and about 20 to200 μm, respectively.

[0265] (3) Optical-Electric Printed Wiring Board Fabrication Method

[0266] The method of stacking the optical wiring layer 11 on theelectric wiring substrate 12 by using the conductive projections will beexplained below with reference to FIGS. 27F to 27J.

[0267] First, as shown in FIG. 27F, a plurality of conductiveprojections 14, 15, 16, and 17 are used as guides for stacking theoptical wiring layer 11 on the electric substrate 12 by aligning them.That is, the optical wiring layer 11 and the electric substrate 12 areso stacked that the conductive projections 14, 15, 16, and 17 extendthrough the through holes 41 in the film to reach the opposite surfaceof the optical wiring layer 11. It is desirable that the optical wiringlayer 11 and the electric substrate 12 be completely adhered by formingan adhesive layer 47 by coating between the optical wiring layer 11 andthe electric substrate 12.

[0268] Next, as shown in FIG. 27G, a thin metal film 30 is formed bysputtering on the surface of the stacked optical wiring layer 11.

[0269] As shown in FIG. 27H, a photoresist 31 is formed by coating.Exposure and development are then performed following the conventionalprocedures to form a photoresist opening 35 for mirror formation.

[0270] Subsequently, as shown in FIG. 27I, an opening 36 is formed inthe thin metal film 30 by etching, and a metal mask for mirror formationis formed. In addition, the substrate is inclined 45°, and a mirror 115is formed by RIE dry etching.

[0271] Finally, projecting partition walls 53 shown in FIG. 27J areformed in desired positions by the following procedure. First, a 60-μmthick photosensitive dry film is closely adhered, and the conventionalexposure and development are performed to form a photoresist opening forframe formation. The thin metal film 30 is used as a cathode to performcopper plating, and the resist is removed. The frame height can becontrolled by the dry film thickness and the plating time. Finally, thethin metal mask is dissolved away to complete the optical-electricprinted wiring board having the projecting partition walls 53.

[0272] The projecting partition walls 53 can also be formed by a thickinorganic film using a green sheet or from a heat-resistant photoresist,in addition to a plated metal. Either partition wall can be easilyformed by the conventional photolithography. The frame shape isdesirably matched with the shape of a terminal of an optical part.

[0273] Another method of fabricating conductive projections is toperform laser beam irradiation or dry etching, from the optical wiringlayer side, for appropriate positions on the optical-electric printedwiring board. In this fabrication method, the electric interconnectformed on the substrate 12 functions as a stopper, so the via hole(opening 36) can be formed to this depth. A conductive projection can beformed by filling this via hole with a metal by plating.

[0274] (4) Printed Circuit Board Fabrication Method

[0275] The method of fabricating a printed circuit board obtained bymounting an optical part including a light emitting laser on theoptical-electric printed wiring board 64 according to the thirdembodiment will be described below.

[0276] First, solder balls are placed inside the partition walls 53, andthe terminals 221 of an electric part (a laser or photodiode) arelightly inserted inside these partition walls 53. Assume that the shapeof the partition wall 53 is a circle 20 μm in width and 80 μm in radius,and the depth of the partition wall 53 is 50 μm. Note also that thenumber of conducting terminals 221 of an optical part is four, and theshape of each terminal is a circle 75 μm in radius. Terminals for anelectric part (a CPU or memory) formed where necessary are placed onslightly soldered metal pads.

[0277] According to the experiment conducted by the present inventors,when the device was left to stand in a reflow furnace at a temperatureof 250° for 10 seconds and then cooled, the terminals of an optical partwere fixed in equilibrium positions determined by the shape of the frameand the surface tension of the molten solder, and the optical axis ofthe laser fell within the range of the central position of the mirror ±3μm. When the terminals of an optical part were placed on flat metal padshaving no recess similar to an electric part, the fixed positions of theoptical part were unstable, and errors of about ±50 μm were produced.Accordingly, the partition walls 53 ensure highly accurate alignmentbetween the electrical conduction and the mirror for optical wiring.Also, the partition walls 53 widen the area of the electrical conductionbetween the conductive projections and the optical part. This canfurther improve the reliability of the electrical connection.

[0278] The following effects can be obtained by the above-mentionedarrangement.

[0279] First, the optical wiring layer 11 is formed on the substrate 12having electric interconnects, so high-density mounting orminiaturization is possible.

[0280] Second, the relative positional relationship between the patternof the core 111 including the mirror 115 in the optical wiring layer 11and the individual conductive projections for mounting an optical partcan be matched with the intended one with extremely high accuracy.

[0281] Third, each terminal of an optical part is accuratelyaccommodated in a recess formed by the partition wall 53. Thisfacilitates optically aligning the optical axis of the optical part withthat of the optical interconnect. Therefore, an optical part and anelectric part can be automatically mounted at the same time.

[0282] Fourth, when an optical part or an electric part is to besoldered, the part is directly connected to conductive struts formed byplating. Therefore, the reliability of the connection improves withoutany influence of solder melting heat. At the same time, the reliabilityof the connection with the electric interconnects on the substrate alsoimproves.

[0283] The present invention has been described above by way of itsembodiments. However, those skilled in the art can reach various changesand modifications within the scope of the idea of the present invention.Therefore, it is to be understood that those changes and modificationsalso belong to the range of the present invention. For example, thepresent invention can be variously changed without departing from thegist of the invention, as indicated below.

[0284] In each embodiment, a mirror formed in an optical interconnect isso formed that the incident angle of light propagating in the opticalinterconnect is 45°. However, this is for the sake of simplicity ofexplanation, so the angle is not limited to this one. Accordingly, whena mirror by which the incident angle is a different angle is formed, anyarbitrary light propagation path can be formed by performing designingin accordance with the angle (e.g., the positions of individualconductive projections are properly matched with the angle).

What is claimed is:
 1. An optical-electric printed wiring board comprising an electric wiring substrate having an electric interconnect, and an optical wiring layer stacked on said electric wiring substrate and having a surface on which an optical part is mounted, wherein said optical wiring layer comprises: a core for propagating light; a clad for sandwiching said core; and a mirror for reflecting light propagating in said core toward said optical part, or reflecting light from said optical part into said core, and said electric wiring substrate comprises: conductive setting means which is a conductor column extending through said optical wiring layer in the direction of stacking and having an end face on which said optical part to be mounted is set, said conductive setting means obtaining electrical conduction between said optical part to be mounted and said electric interconnect.
 2. An optical-electric printed wiring board according to claim 1, wherein the number of said conductive setting means corresponds to the number of electrical connecting terminals of said optical part to be mounted.
 3. An optical-electric printed wiring board according to claim 2, wherein the positions of said conductive setting means correspond to the positions of said electrical connecting terminals of said optical part to be mounted.
 4. An optical-electric printed wiring board according to claim 2, further comprising guide means formed around the end face of each conductive setting means on which said optical part to be mounted is set, said guide means limiting the set position of said optical part by limiting the set position of said terminal.
 5. An optical-electric printed wiring board according to claim 4, wherein said guide means is a conductor and electrically connected to said optical part to be mounted or said conductive setting means.
 6. An optical-electric printed wiring board according to claim 2, wherein a hole for exposing the end face of each conductive setting means is formed in said optical wiring layer, and the set position of said optical part is limited by accommodating said terminal in said hole.
 7. An optical-electric printed wiring board according to claim 4 or 6, wherein the depth of said guide means or said hole is 20 to 200 μm.
 8. An optical-electric printed wiring board according to claim 2, wherein the end face of each conductive setting means projects to a predetermined position from the surface of said optical wiring layer on which an optical interconnect is to be mounted.
 9. An optical-electric printed wiring board according to claim 1, wherein the diameter of the end face of said conductive setting means is 50 to 500 μm.
 10. An optical-electric printed wiring board according to claim 1, wherein said electric wiring substrate further comprises: conductive setting means which is a conductor column extending through said optical wiring layer in the direction of stacking and having an end face on which an electric part to be mounted is set, said conductive setting means obtaining electrical conduction between said electric part to be mounted and said electric interconnect.
 11. An optical-electric printed wiring board according to claim 10, further comprising an electric interconnect on the surface on which an optical part is to be mounted, said electric interconnect being connected to an optical part or an electric part.
 12. A printed circuit board comprising: an optical-electric printed wiring board according to claim 1; and an optical part mounted on said optical-electric printed wiring board.
 13. A printed circuit board according to claim 12, wherein said optical part has a terminal having a shape corresponding to the shape of the end face of said conductive setting means, and connected to said conductive setting means.
 14. A printed circuit board comprising: an optical-electric printed wiring board according to claim 1; and an electric part mounted on said optical-electric printed wiring board.
 15. A printed circuit board according to claim 14, wherein said electric part has a terminal having a shape corresponding to the shape of the end face of said conductive setting means, and connected to said conductive setting means.
 16. A method of fabricating an optical-electric printed wiring board, comprising the steps of: forming conductive setting means on a predetermined electric interconnect of an electric wiring substrate; coating the electric wiring substrate with a first cladding layer; coating the first cladding layer with a core layer; coating a portion of the first cladding layer and the core layer with a second cladding layer to obtain an optical wiring layer; exposing an end face of the conductive setting means from the optical wiring layer; forming an electric interconnect on the optical wiring layer; and forming a mirror in a predetermined position of the optical wiring layer by perforation.
 17. A method of fabricating an optical-electric printed wiring board, comprising the steps of: forming conductive setting means on a predetermined electric interconnect of an electric wiring substrate; forming an optical wiring layer on a support substrate; forming a through hole which allows the conductive setting means to extend therethrough in the optical wiring layer; and peeling the optical wiring layer from the support substrate, and stacking the optical wiring layer on the electric wiring substrate such that the conductive setting means extends through the through hole.
 18. A method of fabricating an optical-electric printed wiring board according to claim 17, wherein the conductive setting means is formed by plating. 